Flexible organic light emitting diodes (OLEDs), such as plastic organic light emitting diodes (pOLEDs), include an electroluminescent organic semiconductor deposited on a flexible substrate. Flexible OLEDs are bendable and lightweight, which allows electronic device manufacturers to explore a variety of designs and configurations that would not be possible with conventional OLEDs deposited on rigid substrates. For example, flexible OLEDs may be incorporated into rollable televisions, flexible lights and foldable mobile electronic devices, such as cellular phones and tablet computers.
The use of plastic in devices that incorporate flexible OLEDs is necessary for lightweight flexible devices; but plastic components are vulnerable to damage and wear. Plastic objects and plastic portions of non-plastic objects are susceptible to physical damage such as scratching, marring, gouging and fracture under mild use conditions. For example, plastic objects may be damaged from interactions with other objects while being carried in a user's pocket or handbag.
Plastics may be protected from damage by applying a protective polymer coating, often referred to as a hard coat. Hard coats may be composed of ultraviolet (UV)-cured acrylates, vinyl ester epoxides, dendritic polymers of polyurethane, dendritic polymers of melamine, or similar polymers and polymer mixtures that form a rigid coating on a substrate. However commercially available hard coats for plastics fail to provide the desired level of protection to flexible plastic articles, such as pOLED displays, due to the processing limitations inherent in applying a coating to a plastic substrate. Hard coats must be applied to plastic articles at low temperatures to prevent damage to the plastic substrates being coated. Low-temperature processing limits the chemistries and processes, which may be employed, which in turn limits the degree of hardness, strength and protective capability of the hard coat. In addition, plastic substrates curl to an increasingly greater extent as films with increasing strain are deposited. Processing conditions during deposition causes strain in the films; these include heat, radiation, and plasma treatment. In general, thicker films and/or harder films (e.g., films with higher oxygen content) cause flexible plastic substrates to curl to a greater extent during processing, for example in a plasma-deposition processes.
Flexible mobile devices that incorporate pOLED displays require hard coats with specific properties in addition to abrasion resistance. Hard coats that are applied to pOLED display stacks must be optically clear and free of optical defects. The frequent use of mobile devices requires highly durable hard coats that maintain their integrity with no permanent plastic deformation for at least 100,000 bending cycles. The construction of pOLED displays, known as pOLED stacks, also imposes requirements on hard coats. The pOLED displays are comprised of multiple layers: pOLED layer, touch layer(s), one or more optical films, and lens layer. The individual layers within a pOLED display can independently be bent to small radii (<5 mm) but once assembled together the stack may break, delaminate, and/or buckle due to tensile and compressive forces arising in the display stack when bent.